Field of the Invention
The invention relates to a device for measuring the location of a leading or trailing edge of a sheet retained on the surface of a cylinder for the purpose of pure measurement, or for determining an actual value within a position-control or regulating system. The invention may be employed in printing machines or other sheet-handling machines, wherein a sheet is transported by means of at least one cylinder.
In rotary printing machines, the precise location and positional stability of the sheet on the surface of a cylinder is decisive for high print quality. The position and locational stability, respectively, of a sheet on a cylinder are dependent upon various factors. A decisive factor is which position the sheet is in when it is taken over by the sheet-holding elements of the cylinder. To that end, in printing machines, front and side lays are provided as stops which are adjustable in order to correct the position of the sheet on the cylinder. Adjustment of the stops can be effected by means of a control device to which measurement signals can be fed regarding the position of the sheet on the cylinder.
After the take-over by the sheet-holding elements, the position of the sheet on the cylinder can be varied even further, due to the processing operation to which the sheet is subjected. For example, when a sheet is imprinted, after the printing nip, sheet lagging occurs oil the blanket cylinder, depending upon the printed image. The tear-off forces which then ensue are propagated as far as the gripper lock, so that gripper slip occurs, which causes an undesired and unintended change in the position of the sheet. As viewed over the width of the sheet, the gripper slip is not constant, so that in order to ascertain the position of the edge of the sheet, it may be necessary to make measurements at several locations. A plurality of measuring locations are also suitable for ascertaining skewing of the sheet and damage to the edge of the sheet.
A measurement device for determining the location of an edge of a sheet on the surface of a cylinder, which is accommodated in its entirety on the cylinder and revolves therewith, has become known heretofore from the German Utility Model DE-GM 77 00 430, for example.
A disadvantage of such a measurement device is that, for continuous display and evaluation of the measurement findings, the measurement signals must be carried onward from the rotating cylinder via rotary leadthroughs or other suitable devices. This requires an additional expenditure of material and finances and a resultant reduction in reliability.
Measuring devices have also become known heretofore which have a photoelectric scanner fixed to a frame and connected to a control device for positioning a sheet at a given location. As a rule, the photoelectric scanner includes a light transmitter and a photoelectric receiver. The light is projected onto the surface of the cylinder and is reflected more-or-less from the sheet, the cylinder surface and regions of the cylinder gap and, by means of an electric circuit arrangement, the position or location of the edge of the sheet in relation to the position or location of a fixed edge, such as a boundary of the cylinder gap, can be determined from the reflected light. For dimensional detection or ascertainment of the position or location of the edge of the sheet, the time that elapses between the appearance of the sides of the signal generated by the fixed edge of the cylinder and those of the signal generated by the edge of the sheet is determined.
A further method wherein the angle through which the cylinder turns between the appearance of these signal edges is disclosed in the published German Patent Document DE 23 22 194 C2 and the published Japanese Patent Document JP 1-306247 (A).
The method of time-measurement is disadvantageous in that measurement errors arise when fluctuations in the angular velocity of the cylinder occur within one revolution. The second method has the disadvantage that the resolution of the incremental angle encoders used for angle measurement is too low and, if the angle encoders which are coupled with the rotation of the cylinder have high resolution, the signal processing speed in the evaluation of the angle encoder signal is limited, respectively, so that the maximum possible rotary speed of the cylinder may be reduced.
To control the measuring operation, it has become known heretofore from the published German Patent Document DE 38 36 310 A1 to use the signal of a digital angle measuring system which is coupled with the rotation of the cylinder. At given angular positions, predetermined switching events are derived, which are controlled by the signal processing electronics for the position measuring signals generated by a scanner. To that end, the signals of the angle measuring system must be prepared or made-ready by circuitry means or by computer means with the aid of a program, which involves additional effort and expense and limits the maximum measurement speed.